This invention relates to compositions and processes for improving brightness and optical properties in pulp and paper manufacture. More particularly, this invention concerns compositions, which in the presence of optical brightening agents effectively enhance the brightness and optical properties of a paper product.
Pulps produced by either mechanical or chemical pulping methods possess a color that can range from dark brown to creamish depending on the wood type and defibering process used. The pulp is bleached to produce white paper products for a multiplicity of applications.
Bleaching is the removal or chemical alteration of those light-absorbing components of unbleached pulp such that the amount of light they absorb after bleaching is greatly reduced. In the bleaching of mechanical pulp, the objective is to decolorize the pulp without solubilizing the lignin. Either reducing (e.g. sodium hydrosulfite) or oxidizing (e.g., hydrogen peroxide) bleaching agents are usually used. The bleaching is often a multistage process. The bleaching of chemical pulps is an extension of the delignification that started in the digestion stage, thus comprising lignin removal and chemical alteration. The bleaching is often a multistage process, which stages may include chlorine dioxide bleaching, oxygen-alkaline delignification, and peroxide bleaching.
In contrast to bleaching, application of optical brighteners changes the appearance of the paper not by altering the light absorbing materials, but instead by adding to the paper additional materials that fluoresce light in their own right. The industry invests significantly in chemicals such as bleaching agents and optical brighteners that improve optical properties of the finished paper or paper products. However, both approaches have their limits: it becomes uneconomical to bleach pulp beyond certain limits and it also tends to degrade and decrease mechanical properties of the resulting paper. Also excessive use of optical brighteners results in economic losses and in undesirable color changes. These two problems limit the degree of brightness improvement that can be achieved using conventional means.
Thus there is a clear need for and utility in a successful and practical solution to increase brightness paper made of bleached pulp. One of the approaches is to increase efficiency of optical brighteners. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists.